a) Never in the current card pool. I used to consider it, but since Enchanter's Wardstone and Enchantment Transfusion, the chance of spending a big chunk of mana on something that whiffs is too great, particularly for the spellbook cost for everyone besides the Wizard. Enchanter's Wardstone doesn't make them whiff, but it can make them much less efficient. In particular, Purge Magic is pretty niche anyway. It only helps if your opponent stacks a ton of enchantments on one creature, rather than spreading them out. Destroy Magic may end up being more generally worthwhile, as it would help against a variety of builds, and it's more likely that positioning could prevent someone from saving the enchantments with Enchantment Transfusion or countering it with Nullify/Reverse Magic.
b) If I was going to run one of those, I would probably decrease the number of Dispels I included by 1.
c) I would run fewer enchantments in builds that I planned to use Purge Magic or Destroy Magic in. Of course, most of my builds are fairly enchantment reliant at the moment, which is part of why I don't find them particularly appealing.
d) At the moment, the Wizard can try it, but it seems pretty expensive for anyone else. The Warlord could try to get by without enchantments and use one of these, but it's so expensive, and for builds that are incredibly reliant on enchantments, there are ways to make this pretty inefficient. I guess the Necromancer can either go with anti-enchantment tech or with curse spam, so there is another mage that doesn't need to rely heavily on enchantments.
e) Enchantment Transfusion and Enchanter's Wardstone, plus trying to use Purge Magic/Destroy Magic decreases the value of putting out enchantments yourself. Enchantments are some of the most flexible investments in the game, with the ability to bluff your opponent and invest a small amount of mana for an effect later, once you have more mana, which can help with improving action efficiency. The possibility of efficiently destroying multiple enchantments comes at the cost of giving up some or all of this action efficiency yourself. In addition, Purge Magic and Destroy Magic are quite expensive, and using them requires you to build up mana ahead of time.
e1) Dispels are cheap and flexible, don't interfere with an enchantment plan of your own, can be used for most enchantments on the fly, and it's not such a big deal if they get countered. Enchantment Transfusion can't be used to move some of the more expensive enchantments anyway, like Forcefield and Mind Control. Plus Enchanter's Wardstone doesn't hurt their efficiency nearly as badly.
f) If I know that they can't counter it or make it whiff with a Nullify, Reverse Magic, or Enchantment Transfusion, they have no Enchanter's Wardstones out, and I don't have any enchantments that will be destroyed when I cast it, then I would consider using Purge Magic once it will destroy at least 12 mana of face up enchantments, but hopefully more like 15+. With Destroy Magic it would be more like at least 16 mana, but preferably 20+.
1 and 2) This point has been raised. I agree that there should be counters to things outside of the school, and that the ways to deal with enchantments are too centralized to the arcane school at the moment.
Take a look at Harshforge Monolith from FiF. That will make the Warlord much stronger against enchantment heavy builds, and will work pretty well against both Enchantment Transfusion and Enchanter's Wardstone. When FiF comes out, I will be building an Anvil Throne Warlord. Maybe I'll try running a Destroy Magic. It will be hard to justify the spellbook points, but we'll see. I can see it being pretty effective in the mid or late game, once I've destroyed any Enchanter's Wardstones.
http://www.allusgeeks.com/2014/04/teaser-tuesday-harshforge-monolith/
